Power Supply 24VDC LEDS controlled by a transistor, what transistor?

Hello can anyone advise on a transistor capable of control an LED array (on/off) that draws 240mA, by a transistor controlled by a arduino board and the array powered by 24VDC? Transistor specs: 24V and minimum 240mA, Thanks. Or where to look for.

Edit, the BC547 have a max of 100mA right?

https://plus.google.com/photos/105013813976245054396/albums/5911246675352005057?authkey=CMeFisuK9MW4Dw

Look at the TIP120. 60Volts and 5 amps max.

Thanks i think will do the work, :)

What about the mosfet IRF520 will do the trick or is something different ?

if you want to use a mosfet transistor, look for a logic level mosfet . Threshold voltage of 4V for the irf520 doesn't mean it will have the lowest "RDS On " value at 4V .

IRF520 is not a logic level mosfet, so not really suitable - although for switching just 240mA it would probably work. However, BC337 (a low-cost NPN bipolar transistor) is suitable and less expensive. Use a base resistor of 220 to 330 ohms.

If you were making a PCB then there are lots of SMD mosfets that would do the job really well.

The BC337 is rated at 1/2 amp. I would be a bit shy to use it for a 1/4 amp load, without some heat sinking.

Can you give us a URL to your LED array?

jackwp: The BC337 is rated at 1/2 amp. I would be a bit shy to use it for a 1/4 amp load, without some heat sinking.

The BC337 is rated at 800mA and 625mW power dissipation. Given a reasonable base drive current (see my suggestion for base resistor), there is NO WAY it requires heatsinking to switch a mere 240mA.

I won’t argue with that. I know you have lots more knowledge on that subject than I do. Evidently I was confused.

Thanks for giving way graciously. For the benefit of others, here is my reasoning, with reference to the datasheet at http://www.fairchildsemi.com/ds/BC/BC337.pdf:

  1. The datasheet quotes Vce(sat) @ Ic = 500mA, Ib = 50mA as 0.7V maximum. Therefore, if we could supply 50mA base current, we could guarantee that when passing 500mA, the power dissipation would be no more than (0.7 * 500) = 350mW, which is well within the 625mW (at 25C ambient) rating. However, we can't supply 50mA base current from an Arduino pin. So 500mA is best treated as a limit that we can't reach.

  2. The datasheet quotes hfe as 60 minimum @ Vce=1V, Ic=300mA. Therefore, if we can tolerate a voltage drop of 1V, then with only 5mA base current it is guaranteed that we can switch 300mA with a power dissipation of no more than 300mW - again well within the rating @ 25C ambient.

  3. If we want to switch 300mA and provide base current of more then 1/60 Ic but less then 1/10 Ic, then we can expect a lower Vce than 1V. Unfortunately, we cant make any guarantees in this area. I am reasonably confident that for any BC337 that meets its guaranteed specifications, it could switch 400mA with 20mA base current and significantly less than 1V drop. But I have to confess that this isn't guaranteed by the datasheet.

In summary, I am comfortable using a BC337 to switch up to 400mA from an Arduino, using 20mA base current. Most BC337s will comfortably switch much more than that, but I prefer to design for the worst-case. Above 400mA, there are better BJTs such as ZTX851.

Having said that, if I am making a PCB then I will take advantage of the many low-cost mosfets in SOT23 packages that are available, and probably use a mosfet to switch anything above 100mA.

jackwp:
The BC337 is rated at 1/2 amp. I would be a bit shy to use it for a 1/4 amp load, without some heat sinking.

Can you give us a URL to your LED array?

My intention is to place the transistor between the 24vdc and the array

Screen Shot 2013-08-12 at 15.53.42.png

A modern logic-level MOSFET is a good choice, something rated at 40V+ and around 10mOhm Rds(on) is a useful device to keep a few of for just such an occasion (and can switch a lot more current should you ever need to). The IRF520 is truly ancient technology these days BTW, and would need a level shifting circuit to provide the 10V gate drive.

paulorcrs: My intention is to place the transistor between the 24vdc and the array

In that case, you need to use high side switching. This requires a level-shifting device, for example a second transistor. You could use am NPN transistor (e.g. BC337 or BC547) to do the level shifting, followed by a BC327 (PNP transistor) to switch the load. See for example http://rayshobby.net/blog/wp-content/uploads/2011/10/remote_switch_interface_schematic.png for the basic arrangement. R2 is not needed in your application, and adding a resistor of about 1K between the base and emitter of the BC327 would be good practice.

Placing the transistor between the array and ground is simpler because you need only one transistor.

MarkT: A modern logic-level MOSFET is a good choice, something rated at 40V+ and around 10mOhm Rds(on) is a useful device to keep a few of for just such an occasion (and can switch a lot more current should you ever need to).

I agree in principle, and if I were designing a PCB then a mosfet would be my choice too. But when building a circuit with through-hole components, there are unfortunately very few mosfets available for this sort of current (240mA). The choice is between power mosfets in TO220 packages, or a few IPAK power mosfets, or the Zetex E-line package mosfets. These all cost 10x more than a medium-current BJT. So for the hobby market, I think a BJT is generally the best choice for switching up to about 400mA.

So, if he's switching high side then he could use a 2n2222a with a base R of 1k, the collector with a 390 ohm driving the base of a 2n2906a. Now if he can change his design to use a common anode array of LEDs, then (as I think dc42 said) a logic level mosfet is the best way. JAMECO has RFP12N10L for .65/pc. 100v and 12 amp.

jackwp: Look at the TIP120. 60Volts and 5 amps max.

TIP41 6 amps, lower voltage and cheaper.